A naked cat
You should use a meter stick to measure the length of a soccer pitch.
I would use a centimetre to measure the length of a boy. Not sure what a boy yard is, though.
Length is measured in meters in the metric system.
Microscope
Meters if you're using the metric system. Otherwise use feet.
inches or centimeters
You don't measure with length. You measure the length of an object with a ruler. You would use inches or metres as your units.
use a ruler to measure the sides then do length times width
we don't use length. we use liter.
Any sensibly sized unit of length, such as feet, yards or metres.
If you live in the United States, you should use inches. If you live outside of the US, you should use centimetres.
The answer depends on whether you wish to measure its mass, its length, its thickness or another attribute.